1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly to light emitting devices and methods of fabricating light emitting devices with integrated drive electronics.
2. Description of Related Art
Light emitting diodes and laser diodes are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (LEDs). Light emitting devices generally include a p-n junction formed in an epitaxial (epi) layer such as gallium nitride (GaN) grown on a substrate such as sapphire (Al2O3), silicon (Si), silicon carbide SiC), gallium arsenide (GaAs) and the like. The wavelength distribution of the light generated by the LED depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that include the active region of the device. Commercial high-efficiency LEDs are typically fabricated from two classes of III-V semiconductor materials. Group-III nitride (III-N) based materials are used for the color range from ultraviolet to blue-green, and Group-III arsenide-phosphide (III-AsP) for yellow to near-infrared.
There has been a great deal of recent interest in LEDs formed of Group-III nitride based material systems because of their unique combination of material characteristics including high breakdown fields, wide bandgaps (3.36 eV for gallium nitride (GaN) at room temperature), large conduction band offset, and high saturated electron drift velocity. The doped and active layers are typically formed on a substrate that can be made of different materials such as silicon (Si), silicon carbide (SiC), and sapphire (Al2O3). SiC wafers are often preferred for these types of heterostructures because they have a much closer crystal lattice match to Group-III nitrides, which results in Group III nitride films of higher quality. SiC also has a very high thermal conductivity so that the total output power of Group III nitride devices on SiC is not limited by the thermal resistance of the wafer (as is the case with some devices formed on sapphire or Si). Also, the availability of semi-insulating SiC wafers provides the capacity for device isolation and reduced parasitic capacitance that make commercial devices possible.
LEDs, which are current-sinking devices, typically require a constant-current, direct-current power supply for efficient and stable operation (e.g. to provide and maintain output intensity and color). Constant-current power supplies capable of matching the voltage supply of an electronic system to the required voltage of an LED can take many forms, but they require circuits composed of active components (e.g., transistors, oscillators, operational amplifiers) as well as linear and non-linear passive components (e.g. diodes, thermistors, inductors, capacitors, resistors). For high efficiency, the input voltage should be matched as well as possible to the operating voltage of the LED(s).
The power sources typically available within electronic devices are voltage sources such as batteries, fuel cells, or rectifying or switching power supplies to provide DC voltage from an AC mains supply. The LED circuits typically consist of one or more transistors, oscillators, or amplifiers connected to inductors and capacitors to provide the constant current. The power supply (source) and current control components in current LED technologies are physically separated from the LED, and these technologies suffer from one or more of the following disadvantages with respect to power sources and control components:
(a) the voltage of the source is not typically matched to the operating voltage of the LED;
(b) the operating voltage of the LEDs, even those originating from the same manufacturer and of the same part type and lot, are not usually matched well enough to allow for voltage source operation;
(c) the realization of drive circuitry external to LEDs can be problematic for potential LED users who may be unfamiliar with the design of constant-current circuits; and
(d) applications in which space is limited cannot allow for power supplies external to the LED.